Works in ointment to stop case of fleas (9)
I believe the answer is:
functions
'works' is the definition.
(function is a kind of work)
'ointment to stop case of fleas' is the wordplay.
'ointment' becomes 'unction' (I've seen this before).
'to stop' is an insertion indicator (stop can mean to plug a hole in something).
'case of' says to hollow out the word (remove centre letters) (like emptying a case).
'fleas' with its centre removed is 'fs'.
'unction' placed into 'fs' is 'FUNCTIONS'.
'in' is the link.
(Other definitions for functions that I've seen before include "The uses of something" , "parties" , "Works" .)